Magnetic Flux Leakage (MFL) is a widely used nondestructive testing (NDT) method for assessing the condition of aboveground storage tanks. By utilizing the magnetic properties of steel, MFL provides a reliable way to detect corrosion and defects on tank floors. However, to fully harness its potential, it is essential to understand the theory, limitations, and practical applications of this technique.
This article explores the principles of MFL, the challenges encountered in inspecting tank floors, and the tools and techniques that make it effective.
Understanding Magnetic Flux Leakage
At its core, MFL introduces a strong magnetic flux into a steel surface, such as a storage tank floor. Magnetic lines of force, or flux, naturally prefer to travel through steel rather than air. When there is a reduction in steel thickness—caused by corrosion or physical defects—some of the magnetic flux “leaks” into the air.
This leakage is detected by sensors near the inspection surface, which interpret the flux leakage as an indicator of material loss. The two primary types of sensors used in MFL are:
- Coils: Passive devices that follow Faraday’s Law, generating voltage as they move through a magnetic field.
- Hall Effect Sensors: Solid-state devices that measure the absolute flux density and convert it into a voltage signal.
While Hall effect sensors are highly sensitive, coils are typically preferred for aboveground storage tank inspections because of their reliability and stability in challenging field conditions.
MFL as a Qualitative Tool
One of the defining characteristics of MFL is that it is primarily a qualitative inspection tool. While it reliably identifies areas of material loss, it cannot precisely measure the remaining thickness of the steel. Signal amplitude, or the strength of the leakage detected, is influenced by the volume of material lost rather than the actual thickness of the remaining wall.
To obtain accurate, quantitative data, MFL is often complemented with ultrasonic testing, which provides precise measurements of wall thickness and helps verify MFL findings.
Challenges in Inspecting Aboveground Storage Tank Floors
Inspecting the floors of aboveground storage tanks presents unique challenges compared to other applications, such as pipelines or tubular structures. These include:
- Uneven and Corroded Surfaces:
Tank floors are rarely flat and are often affected by corrosion, debris, or standing water. These factors introduce noise into the MFL signals and can reduce sensitivity. Proper cleaning—such as water blasting and removing loose debris—is essential for accurate results. - Limited Coverage:
Achieving 100% coverage of a tank floor is difficult due to physical access restrictions, particularly near lap joints and tank shells. Smaller scanning heads and flexible equipment design are necessary to increase coverage in confined spaces. - Differentiating Defects:
MFL cannot inherently distinguish between defects on the topside and bottom side of the tank floor. However, topside defects often produce lower amplitude signals, allowing them to be “tuned out” during analysis. Visual inspection techniques are also effective in addressing this limitation. - Environmental Challenges:
Temperature extremes, dust, moisture, and other environmental factors can affect equipment performance. Inspection tools must be rugged and field-tested to withstand these conditions.
Advances in Equipment and Techniques
Modern MFL equipment is designed to address many of these challenges, emphasizing adaptability, precision, and reliability. Key considerations include:
- Sensor Selection: Coils are generally preferred for their resilience to surface imperfections and consistent performance.
- Magnet Design: Permanent magnets are lightweight and compact, ideal for portable systems, while electromagnets allow adjustable flux levels but are bulkier.
- Data Visualization: Computerized mapping of flux leakage signals provides a graphical representation of defect areas. While mapping is limited to qualitative information, it is valuable for documentation and correlating data with ultrasonic results.
Integrating Ultrasonic Testing
Because MFL is qualitative, it is often paired with ultrasonic testing to achieve a comprehensive assessment of a tank floor. Ultrasonics excels in providing precise wall thickness measurements and can verify MFL findings, ensuring accurate identification of critical areas.
This hybrid approach offers a balance of speed and precision, making it an essential practice for modern inspections.
The Role of Operator Training
Effective MFL inspections depend not only on advanced technology but also on skilled operators. Training programs must address:
- Corrosion evaluation techniques.
- The proper use of ultrasonic testing for prove-up.
- Interpretation of flux leakage signals.
Certification ensures that operators are qualified to perform inspections, reducing the likelihood of errors and improving overall reliability.
Conclusion
Magnetic Flux Leakage remains one of the most reliable and efficient methods for assessing the condition of aboveground storage tank floors. While its qualitative nature limits its ability to measure wall thickness directly, integrating MFL with ultrasonic testing provides the precision needed for comprehensive evaluations.
Understanding the principles of MFL, addressing environmental and design challenges, and ensuring proper operator training are all essential for maximizing the effectiveness of this technique. By advancing both the technology and the expertise behind MFL inspections, we can ensure safer and more reliable results for the long-term maintenance of storage tanks.